Field of the Invention
The present invention relates to an inorganic polarizing plate, and a production method thereof.
Description of the Related Art
A polarizing plate, which is configured to polarize light in a certain direction, and only pass the polarized light through, is used in a liquid crystal display.
As for the polarizing plate, a polarizing film, which is prepared by adsorbing an iodine compound on a polyvinyl alcohol (PVA) film, and stretching and orientating the resultant, to thereby exhibit absorption linear dichroism of visible light, has been widely used. Both sides of this polarizing film are sandwiched with transparent films, such as triacetyl cellulose (TAC) to secure mechanical strength, heat resistance, and moisture resistance, and a hard coat treatment is performed thereon to prevent scratches, or depositions of dusts.
Among light incident to the polarizing film, a polarizing component of the light, which is not passed through the polarizing film, is absorbed in the polarizing film, and released outside the polarizing film as heat. In the case where strong light is applied, therefore, the temperature of the polarizing film is increased by the generated heat, to thereby deteriorate the polarizing properties. This is due to heat resistance of the organic material itself, and thus it is difficult to fundamentally solve this problem.
Therefore, a wire grid polarizing plate composed only of inorganic materials has been considered as a polarizing plate usable in an environment where high heat resistance is required. The wire grid polarizing plate has a structure where a wire grid formed of fine metal wires having a frequency equal to or lower than a wavelength of light is formed on a surface of a substrate (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2010-204626, and 2010-117646).
The wire grid polarizing plate causes a phenomenon called “Rayleigh resonance” associated with a pitch (frequency) of the metal wires, because of the principle thereof. The properties of the wire grid polarizing plate significantly change at adjacent to the resonance wavelength, to thereby degrade the properties as a polarizer. The resonance wavelength is determined by the following formula:
      λ    R    =                    p        ⁡                  (                                    n              eff                        ±                          sin              ⁢                                                          ⁢              θ                                )                    k        ≅          pn      eff      
In the formula above, λR is a resonance wavelength, p is a pitch of metal wires, neff is an effective refractive index adjacent to the metal wires, θ is an incident angle, and k is diffraction order. Under the actual usage conditions, θ is typically 0°. The diffraction order indicates simple transmission at 0-degree (k=1), and a loss at the higher-degree diffraction at the first-degree (k>2), hence k=1 (right formula). Accordingly, the resonance wavelength of Rayleigh resonance is proportional to a pitch of the metal wires, and the effective refractive index adjacent to the metal wires. Moreover, the deterioration of the properties due to Rayleigh resonance can be prevented by largely sifting the resonance wavelength from wavelengths of a bandwidth for use.
In order to lower the effective refractive index adjacent to the metal wires, to shorten the resonance wavelength of Rayleigh resonance in a polarizing plate, proposed is a method where a region between the metal wires having a low refractive index is increased along a thickness direction of a substrate to lower the effective refractive index (see, for example, JP-A Nos. 2003-502708, 2010-530994, and 2010-530995, and Japanese Patent (JP-B) Nos. 4763099 and 4800437).
Moreover, it is effective to narrow a pick between the metal wires, in order to shorten the resonance wavelength (λR) in the formula above. An effect of shortening a resonance wavelength is lager especially with narrowed pitch of metal wire than reduction in the effective refractive index, as it can be changed at a lager ratio.
As for a production of the wire grid, conventionally, a method, such as interference exposure, has been used. In order to narrow a pick between metal wires in the interference exposure, it is necessary to shorten a wavelength of light emitted from a laser light source, used as exposure light. However, there is a problem that there is no light source actually used, as it is attempt to make a pitch between metal wires 100 nm.